Abbreviations
    ACK Acknowledgement    ARQ Automatic Repeat-reQuest    CBRA Contention Based Random Access    CCCH Common Control Channel    HARQ Hybrid ARQ    IP Internet Protocol    MAC Medium Access Control    MSG Message    NACK Negative ACK    NAS Non Access Stratum    RACH Random Access Channel    RB Radio Block    RLC Radio Link Control    RLC-AM RLC Acknowledge Mode    RRC Radio Resource Control    TEID    SAE-GW user plane node    TTI Transmission Time Interval    UL Uplink    VoIP Voice over IP
The present invention refers to a system described in the project Long Term Evolution, LTE, which is a further development of 3GPP and is introduced in 3GPP Release 8. LTE uses Orthogonal Frequency Division Multiplexing, OFD; together with advanced antenna technologies. Existing 3GPP (GSM and WCDMA/HSDPA) and 3GPP2 (CDMA2000 1xRTT, EV-DO) systems are integrated to the LTE through standardized interfaces providing optimized mobility with LTE.
LTE is a data packet wireless communication systems for mobile user equipments such as computers and telephones or the like. Communication between the mobile user equipment and a second party in the system is done by routing data packets between nodes over data links in the system. In the following text “uplink” refers to communication from the user equipment to the second party and “downlink” refers to communication from the second party to the user equipment. In both the uplink and downlink direction, a message intended to be transmitted is broken down in the transmitting unit into a number of data packets being routed to and assembled by the receiving unit.
The mobile user equipment communicates over an air interface with a node in the form of a base transceiver station, eNodeB. The data packets are queued or buffered in the eNodeB for downlink communication and in the user equipment for uplink communication. When a buffered data packet is transmitted from the eNodeB or the user equipment, a so called HARQ (Hybrid Automatic Repeat-reQuest) process starts a timer with a set time interval. When the recipient, i.e. the user equipment or the eNodeB, receives the data packet it transmits an acknowledgement signal, ACK, back to the sender that transmitted the data packet and when the ACK is received the HARQ process stops and a new packet can be sent using the same HARQ process. However, if the ACK has not been received within the time interval, the HARQ process considers the data packet to have not been received and therefore signals to the sender to retransmit the data packet. In LTE the method of synchronous HARQ has been chosen against asynchronous HARQ. The motivation has been that since asynchronous HARQ implies that (re)transmissions for a certain HARQ process may occur at any time, the explicit signaling of the HARQ process number is required. With synchronous HARQ, that number can be derived from e.g. the sub-frame number.
In LTE it has also been agreed to use a Stop-and-Wait synchronous HARQ for uplink HARQ. This means that (re)transmissions for a certain HARQ process are restricted to occur at known time instants, in between which the sender stops and waits for ACK/NACK feedback from the receiver.
The queued or buffered data packets together with the above described Stop-and-Wait synchronous HARQ process result in a delay in the form of latency. Latency in a packet-switched network is normally referred to as a round-trip latency and is measured as the time for a one-way latency from source to destination plus a one-way latency from the destination back to the source, i.e. from the moment a packet is transmitted until the ACK has been received by the sender. The present invention refers to a non-trivial network, where a typical packet will be forwarded over many links via many nodes/gateways, each of which will not begin to forward the packet until it has been completely received. The minimal latency is the sum of the minimum latency of each link, plus the transmission delay of each link except the final one, plus the forwarding latency of each gateway.
Improved latency for connection establishment is an essential target of LTE standardization. As a result it is now the agreed assumption that certain Non Access Stratum, NAS, messages must either be concatenated with RRC messages or carried within RRC for its speedy transportation.
The eNodeB controls all active user equipments in a geographical area called a cell. When a user equipment is in an idle state, or is new in the cell, the eNodeB is not aware of the user equipment. When the user equipment wants to transmit uplink data packets, the first step is to become active, i.e. to become known to the eNodeB. The eNodeB always broadcasts system information comprising information about preambles. The user equipment becomes active by use of a Contention Based Random Access, CBRA, process or a Contention Free Random Access, CBRA, and the process starts with the user equipment transmitting to the eNodeB a Random Access Preamble comprising information about the user equipment. The eNodeB transmits a response in the form of a Random Access Response. After this the first scheduled transmission, hereinafter called MSG3, uses N-channel Stop-and-Wait synchronous HARQ as described above. Each HARQ (re)transmission adds incremental redundancy which the receiving side can use for soft combining to secure reception.
Since MSG3 is the very first L3 message, i.e. request to setup Radio Resource Control, RRC, it can not rely on Radio Link Control, RLC for segmentation since it must inevitably be sent on a logic Common Control CHannel CCCH which uses Radio Link Control Transparent Mode, RLC-TM. Thus, the only segmentation offered to the transmission of MSG3 is the one achieved by HARQ and therefore within MAC alone.
It is a result of the assumptions in 3GPP TR 36.300 V8.2.0 (2007-10) that the MSG3 size must be fixed. The current estimation is that MSG3 must be restricted to some 72 or alternatively 96 bits to achieve worst case coverage for cell border user equipments. In case the initial NAS upload message is larger than what such an MSG3 can provide, it needs to be split in two parts where the latter part is sent after CBRA has been successfully completed.
The problem here is multifold and needs to be addressed in its larger context.
Firstly, MSG3 needs to be of varying size because the CBRA procedure is a multi-purpose procedure and the required MSG3 size really depends on the purpose of each procedure that has triggered CBRA to occur, i.e. which content needs be transferred by each separate initial user equipment message. It is a waste of resources to have one fixed size.
Secondly, the latency must be optimized for certain scenarios while for other scenarios it does not need to. For cases which are not time-critical, there is no need to add such content to MSG3 which is not required for the proper user equipment identification, but rather that can follow after the CBRA procedure has been completed. Again, it is a waste of resources to have one fixed size.
Thirdly, the size of MSG3 must not be restricted so that messages that apply to time-critical scenarios would not fit. The connection establishment as described above is the main example of such a time-critical scenario. Here MSG3 must, for the optimal latency, house as Initial UE message one RRC Connection Request (which is terminated in the RAN layer) and one NAS: Service Request (which is terminated in layers above RAN).
Finally, only few bits of information have been exchanged at the time of MSG3 and moreover those have been exchanged over a common channel. The user equipment has with the Random Access Response received a fixed resource assignment in the time-frequency domain for MSG3 transmission. It has not learned much of which exact power is the adequate one to reach the RBS and must use approximately the same power it used to transmit the preamble. Since, according to the current 3GPP assumptions, the resource is a fixed-size one, the user equipment (UE) which has the worse channel to reach the RBS will need to retransmit more times than that with a better.
While the method of N-channel Stop-and-Wait Synchronous HARQ serves its purpose as an efficient method to secure uplink transmission whenever a connection has been established, it fails as an efficient method for the most swift and secure establishment of such a connection. The Stop-and-Wait method ensures that incremental redundancy is added to the transmission, but at the cost of an approximate order of one round-trip delay alongside each retransmission. This in turn limits the maximum size of MSG3. It is the current 3GPP assumption that the secure transmission of MSG3 requires in average an approximate total of 3-4 transmissions which in turn limits the MSG3 size to some 72-96 bits. Needless to say, 3GPP has for a lengthier time been and is still in a lengthy deadlock debate, involving complex inter-work between several RAN groups, as well as towards CT and SA groups, how to squeeze the content of MSG3, with the risk of loosing functional content.
It is therefore an object of the invention to find a faster setup of a communication link in a Random Access process allowing an adjustable size of the MSG3.